I understand your point (and I personally want to see Emma again) but this is set 10 YEARS after First Class. A LOT can happen in ten years. Her absence would be understandable and it does nothing to detract from the series as a whole in terms of continuity. They could very well write her out of the story with a line. Besides Emma doesn't appear in the original trilogy at all so we wouldn't necessarily need to know what happened.

Yes Magneto broke her out of prison and offered her a place on his team but again this is ten year's later she could have served the purpose that he wanted her for. And again he could explain that with one line of dialogue.

I think if Emma isn't in this then Fox might say she is on a mission researching stryker, and that's where she gets captured and Wolverine comes save her. To fix the problem with XMO wolverine. or maybe she's dead

I think it would be nice to address her absence, if indeed she is absent. That's because it was in one of the First Class final scenes in which Magneto recruited her, and so if you immediately watched DoFP you would want to know something of what happened in those 10 years.

The First Class series was supposed to fill in the backstory of the missing years not create even more mysteries about those missing years.